In the automotive vehicle field there is an ongoing need for reducing weight of vehicle components. Traditionally, many vehicle body components have been made from steel. Joining such parts typically has been done by welding. In recent years, vehicle manufacturers have sought to substitute other materials for steel. For example, due to its relatively high strength to weight ratio, aluminum has been examined as a substitute material. When steel and aluminum are employed in combination, unfortunately, welding is an impractical solution. There is thus a need for forming a relatively high integrity joint between steel and aluminum for such applications.
One particular application that involves forming joints between steel and aluminum is the formation of vehicle roof structures. For these structures, efforts have been made to join an aluminum roof panel structure to a steel frame structure using rivets. When aluminum is attached to steel using rivets as mechanical fasteners, and the assembly is subjected to thermal cycling (such as under paint bake conditions), there results a potential for distortion of the assembly due to differing coefficients of thermal expansion. For instance, the aluminum experiences a bowing effect that increases any gap that may exist between aluminum and steel components.
Efforts have been made to employ pumpable adhesives between steel and aluminum. By their nature, pumpable adhesives tend to have a relatively low viscosity. As a result, when employed between steel and aluminum, there is a propensity for the steel and aluminum to come into contact with each other during riveting. This creates the potential for galvanic corrosion at the points of contact. Such adhesives also have made it necessary is some instances to employ secondary applications of sealant for assuring moisture protection at the joints.
What is needed is an adhesive and system for applying it that has characteristics sufficient for allowing riveting while still maintaining a separation between the steel and aluminum components during riveting.